Optical coherence tomography (“OCT”) is a technique for obtaining high resolution cross-sectional images of biological tissues. Clinical OCT studies conducted in the gastrointestinal tract and cardiovascular system have shown that OCT is capable of providing images of the architectural (>20 μm) microanatomy of a variety of epithelial tissues, including the layered structure of squamous epithelium and arterial vessels. However, for certain medical applications, such as the early detection of high-grade dysplasia in Barrett's esophagus and the identification of inflammation within atherosclerotic plaques, visualization of structures that are on a size scale of <20 μm may be preferable. OCT systems, with typical axial resolutions ranging from 8-12 μm, have the potential to resolve many of these structures, including nuclei, individual glands, and macrophages. Unfortunately, speckle, which occurs on the same size scale as these features, may prohibit unambiguous including nuclei, individual glands, and macrophages. Unfortunately, speckle, which occurs on the same size scale as these features, may prohibit unambiguous identification of the cellular and subcellular tissue components required for widespread clinical utilization of such technology.
Catheter or endoscope access and high-speed imaging is used in order to perform OCT in the internal organs of patients. In order to minimize diameter, most catheter-based OCT probes employ a single optical fiber to illuminate the sample and detect the signal from the tissue. High frame rates (typically 4-10 frames per second) are preferred for performing OCT imaging while minimizing artifacts caused by patient motion. A way to reduce speckle in OCT images that does not significantly increase the complexity of single optical fiber probe designs while maintaining high frame rates may be beneficial for applying OCT to accurately detect and quantify key microscopic tissue structures in patients.
The reduction of speckle in the OCT images speckle has been previously described. A publication by J. M. Schmitt, “Array Detection for Speckle Reduction in Optical Coherence Microscopy,” Phys. Med. Biol. 42, 1427-1429, 1997, the entire disclosure of which is incorporated herein by reference, describes a procedure for a speckle reduction by averaging multiple images acquired at different angles, known as angular compounding. In this publication, multiple (N) detectors receive images that have been acquired from different angles. The images are averaged incoherently, providing an improvement (√{square root over (N)}) in the signal to noise ratio (“SNR”). While this technique has the advantage that the measurements may be performed in real-time, the experimental apparatus as therein would not be compatible with a single fiber optic catheter.
Another publication, M. Bashkansky and J. Reintjes, “Statistics and reduction of Speckle in Optical Coherence Tomography,” Opt. Lett. 25, 545-547, 2000, the entire disclosure of which is incorporated herein by reference, describes an alternative technique for angular compounding to reduce speckle. In this method, a retroreflector apparatus is translated in front of the objective lens to change the angle of the incident beam on the tissue. N successive images are acquired and added incoherently to reduce speckle, again improving the SNR by a factor of √{square root over (N)}. While this method may be less complex than the use of multiple detectors, the time needed thereby to acquire the images is increased by N. In addition, the implementation of this method within the confines of a small diameter catheter or endoscope could be difficult.